In known technology, the fitting of the injector high-pressure fuel supply conduit has a threaded portion for engaging a threaded seat in the usual hollow body of the injector; and sealing is ensured by a sealing ring normally defined by a metal sealing washer.
In one known injector, the fitting has another portion adjacent to the threaded portion, and which normally has a polygonal section for tightening the fitting inside the threaded seat in the injector; the polygonal-section portion and the threaded portion form a first annular shoulder which, in use, faces another annular shoulder on the hollow body; and the sealing washer is positioned between a bottom surface of the threaded seat and a flat end surface of the threaded portion of the fitting. The two surfaces must therefore be machined accurately so they are flat and perpendicular to the respective axes, with a small amount of surface roughness to ensure high-pressure sealing.
The above known connecting device has various drawbacks. In particular, machining the bottom surface of the seat is extremely difficult on account of its location. Moreover, to achieve a high-pressure fuel seal, a strong fastening torque must be applied to the fitting, thus resulting in a corresponding axial load on the bottom surface of the threaded seat. The bottom surface is located at a thin portion of the hollow body, which also contains the intersection of the holes or conduits supplying high-pressure fuel to the control chamber of the metering valve, and to the injection chamber of the atomizer nozzle of the injector. This portion of the hollow body therefore represents a high stress concentration region, on account of the sharp edges of the hole intersection and the axial stress transmitted by the fitting, so that the fatigue resistance of the hollow body of the injector becomes critical.
The axial load produced by the tightening torque of the high-pressure conduit fitting may also strain the hollow body of the injector in a highly delicate region as regards high-pressure sealing, both on the washer between the hollow body and the fitting, and between the hollow body and the body of the metering valve.